There has been known a device having a honeycomb structure body arranged inside of an exhaust gas pipe, which is capable of purifying exhaust gas emitted from an internal combustion engine mounted on a motor vehicle, etc. In general, the honeycomb structure body has a plurality of cell walls and cells. The cell walls are arranged in a lattice arrangement. Each of the cells is formed by the cell walls. When a high temperature exhaust gas flows in the cells of the honeycomb structure body, catalyst supported by the honeycomb structure body is activated and the activated catalyst purifies exhaust gas. In the inside of the exhaust gas pipe, there is a tendency that a large amount of exhaust gas flows in the center portion in a radial direction of the honeycomb structure body, and a small amount of the exhaust gas flows in an outer peripheral portion of the honeycomb structure body. Accordingly, the flow speed of the exhaust gas is high at the center portion, and is gradually reduced toward the outer peripheral portion of the honeycomb structure body. That is, the flow speed of the exhaust gas varies inside of the honeycomb structure body. As a result, the honeycomb structure body has an insufficient purification efficiency of exhaust gas.
In order to avoid this, there has been developed a honeycomb structure body having a structure in which cell walls are curved by a convex shape from the center portion to the outside or from the outside to the center portion of the honeycomb structure body (see patent document 1). In the honeycomb structure body having the structure previously described, the cell density continuously varies. As a result, this allows the exhaust gas to flow at a uniform flow speed in the overall honeycomb structure body.